1. Field of the Invention
This invention relates to electronic process control systems that use a digital computer and provide for the generation of an analog signal or digital signals that control the operation of devices of process equipment.
2. Description of the Prior Art
U.S. Pat. No. 3,760,374 describes conventional process control systems of the prior art and discloses the system of the patent that is stated in the abstract of the patent as follows: An electronic process control system including a digital computer, a computer interface that is easily replaceable in order to change computer types, a logic module for grouping the computer and controller signals and converting between digital and analog representations, and a control unit interface for generating analog signals for a control unit in response to computer commands and adapting the analog control unit to communicate with a digital computer. The computer interface in the system of that patent is designed for each type of computer and is required to rearrange the computer output into a format required by the invention of that patent. The computer interface receives digital data from the computer by a cable or bus and breaks the data into three digital words. The first word contains the required command instructions. The second word contains the address of one of 16 process control loops, each having a control unit interface. The third word contains the value of the change to be effected. The address word is decoded and the address line of the proper control unit interface is activated. The third or value word is handled either as an absolute position signal or incremental change depending upon the proper command instruction. If incremental, the value is used to generate a series of pulses which are passed on to the control unit interface. If absolute, the value is first related to the present value of the proper signal and the difference is converted into a series of pulses. The control unit interface, that is designed independently of the choice of the computer, is required to convert status and pulse signals to logic and analog voltages. Each control unit interface has a register for accepting the command instructions which are supplied in parallel to all control unit interfaces but only that control unit interface, that is activated, accepts the command instructions. The activated control unit interface converts the series of pulses into an analog value that is stored in a particular memory of that control unit interface for generating a set-point or an output signal. For additional systems of that patent there are additional computer interfaces with associated additional control loops and all computer interfaces are connected in parallel by the cable or bus to the computer and thus are connected to different bits or wires of the bus.